74279 Quad S-R Latch and the Test Fixture
Switches are electronically "dirty". If we take a close look at what happens when
we toggle a switch we understand why. As contacts open they rub against one another for
a few milliseconds before opening. This gives us a bouncing signal on one contact. There
is about 10 milliseconds of delay as the wiper moves from one contact to the other
followed by about 5 milliseconds of bouncing. Again this results in bouncing signals on
the contact. To a circuit that works in nanoseconds these contact bounces are multiple
signals.
For our exercises, and for test fixtures, we need good clean signals, free of contact
bounces. To do this we use an S-R (Set - Reset) Latch. We can make these up of a
NAND or NOR gate. It is likely you have seen this in lessons on basic gates before. It
takes two gates to make an S-R Latch. The output of each gate is tied to the input of the
other gate. The other input goes to our switch. The latch Sets and Clears on the first edge
and dirty signals do not bother us.
The 74279 is four of these S-R Latches in one package. They operate independent
of one another except for having a common power source. (See drawing of the four S-R
Latches)
Actually the 74279 has two latches with a second Set input. Pulling either Set
input Low will Set the latch. When a Set input goes Low the Q output for that latch will
go High. The output will stay High until that latch gets a Low on the Reset input.
The 74279 also comes in various families of TTL and CMOS. Prefixes before the
"74" are specific to a specific manufacturer. Letters between the "74" and the "279"
specify the family characteristics. "L" for Low Power. "S" for Schottky technology". "C"
for CMOS devices, and such. None of this changes the function. They all have the same
pinout. Letters following the "279" specify the temperature range of the part and the case
style of the IC. The internal chip is the same, but it can be put into various case styles.
Pinout may vary between case styles.
The typical schematic symbol for this IC is usually just shown as one big block
and you are supposed to know what all the letters mean because you , once upon a time,
have seen the data sheet and read it.. The schematic symbol may vary from one drafting
software library to another, but it should be recognizable at any rate as long as you are
familiar with how the chip works.
The bubbled inputs indicate that these are Active Low inputs. When the latch is
Set the Q output goes High. Depending on how the IC is used in the circuit it may be
drawn as a single block, or four individual latches similar to that shown on the previous
drawing.
The drawing of the 8-bit test fixture, Driver section, shows an example of the
74279, as we will be using it. We use two of them to give us eight de-bounced switches
as signal sources. At the output we have a ULN2803.
The ULN2803 is eight Darlington Transistor Arrays. The 74279 can only drive
about 16 mA low and only 400 A High. It can only drive to a 5 Volt level. For our
purposes we want a test fixture that can drive to +5, +12, +13, or maybe even +24 Volts.
The ULN2803 can drive up to 50 Volts and at 500 mA of current. This gives us the
driving capability similar to what we would find in a basic game, so we are also
simulating driving capability similar to what we would find in a game.
The ULN2803 looks like this on the inside. (Refering to the drawing of the
Darlington stage.) There are eight of these circuits with a common connection on Ground
and Common.
Other than Ground and Common they are eight separate circuits. The outputs can
handle up to 50 Volts and each output can drive up to 500 mA. The input is designed to
work from a standard TTL signal.
The Common connection can be used two ways. By original intention it is
designed to be a protection diode when driving an inductive load. D2 and D3 protect the
output transistor from Inductive Kick. If we are not driving an inductive load we may
also get a little creative. If we were driving lamp outputs we could use the Common
connection as a Lamp Test feature. Pulling Common Low would turn on all the lamps at
the same time.
This gives us eight clean outputs we will use to exercise other ICs we will study,
or if we are building it as a test fixture, eight clean outputs to manipulate an assembly.
What type of switches we would use depends on our application. For a test fixture we
want good toggle switches. It is an error not to build the test fixture as a circuit less
reliable than the circuit we want to test. The tester has to be more dependable or we will
end up troubleshooting a problem on an assembly that traces back to being a defective
test fixture. For our simple exercises we may want to just use a SPDT DIP Switch. As a
test fixture we would probably want to build this circuit in a box with a good quality
connector. As an exercise add-on we be happy just building it up on a board and letting it
go at that.
Both the 74279 and the ULN2803 are available from Betson Imperial. They can
be contacted at 1 (800) 828-2049. The 74279 is their part number 55-7500. The
ULN2803 is their part number 55-7689. www.betson.com Prices are not listed in their
catalog for these parts. Good catalog, by the way.
Next we need a way to monitor digital outputs for either a test fixture our the
exercise lab. We could use plain LEDs, but this is too limited. Most of the time the
outputs can drive an LED (5 mA to 30 mA). Sometimes not. To be safe we will include a
Buffer. Eight is again a good number. Four has proven to be too few. Many ICs and
assemblies have more than four outputs.
The 74LS244 (Betson Imperial part number 55-7635) is an Octal Buffer Line
Driver. This gives us eight outputs capable of driving an LED just fine, all in one
package. (Refer to the drawing of the 8 LEDs.)
Using the 74LS244 a Low in lights the LED. If you would prefer a High in to
light the LED you can use a 74LS240 as shown as the option above. If we were
interfacing to an IGT circuit runningon 13 Volts we may use a different design, butthis
section can easily be adjusted as the needs suit you.
We now have eight outputs to drive a circuit and eight inputs to monitor outputs
of a circuit. Let's put this to use, shall we? Starting with something useful...
Bally Coin Comparator and COD board tester
You can buy a Coin Comparator tester for around $1,000.00 that tests all kinds of
Coin Comparators, but it doesn't do Coin In Optics or test the cables, and it is these two
later items that fail often. So let's make a test fixture that tests all three: The Coin
Comparator, the COD board and the cables that tie them together. All we need is a wood
frame, a Coin In Handling mechanism taken from a game that holds the Coin Comparator
and the COD block.
Now that we already have the basic Drivers and Receivers in our test fixture, all
we have to design is the cabling to connect to our test fixture using the same connector
that would otherwise go to a game, and power. We need +5 Volts @ about 500 ma or so.
We need +24 V at 500 mA or so.
We have two signals that normally would be coming from the game. "CC Enable"
and "Reset". these signals must now come from two of the Drivers in our test fixture. We
have two signals coming from the circuit being tested that would normally go to the
game. "Error" and "Credit". These will instead go to two of the lines we monitor on our
test fixture.
Power to the test fixture can come from the same +5 Volts. +24 Volts only needs
to go to the circuit being tested.
We can manipulate the COD and Coin Comparator just as the game would. We
can Enable the Coin Comparator, run coins through the Coin Comparator and see that
"Credit" works. We can deliberately do a Coin Jam or reverse Coin and see the "Error"
signal work. We can clear the error condition by pulling on the Reset line. (refer to the
Bally CC and COD drawing)
We can test the Coin Comparator, COD optics as well as those cables that fail so
gosh darn often.
Once we have the test fixture built all we have to do to test an assembly is make
an interface cable (well most of the time). Now let's try something a little trickier. IGT
182 x 16 VFD display 75117700 and 75117800. This is a Netplex device. We can bring it
up under power. See that it comes up and gets through its internal self test and displays a
message. We can give it a Netplex Reset and see that happen. We can pull on the data
line and see the LED come on telling us the Netplex Data In receiver is at least working.
We may not be able to give it a complete functional test, but we can catch most of
the things that fail, mainly being that dreaded capacitor, C3. (Refer to the drawing of the
75117700 connector.)
What more can you ask for from a test fixture you an build for such small bucks?
Four ICs, a small circuit board, a few hours of build time, and hundreds of dollars saved
on repairing assemblies for parts that only cost quarters.
SENET
At the far edge of this circuit's usefulness is using it to emulate a SENET
controller. We assign our outputs to the various SENET drive lines and run Senet Data in
to our monitor inputs. To manipulate the SENET we need to clock in an address of the
device we want to talk to followed by data we want to send to it. As we send data we also
receive data on the Data In lines. It takes a bunch of switch flippings to do a simple
operation and more than a few operations to give a complete test. But by the time you
build the test fixture and write the procedure there are few mysteries to SENET for you.
We can do this with SENET because SENET is a Synchronous Serial scheme and
will work at any speed, even the speed of manual switch flippings where we can control
the Clock line. We can't do this with asynchronous schemes like RS-232 or NETPLEX.
Herschel